1. Field of the Invention
The present invention relates, in general, to duplexer dielectric filters and, more particularly, to a duplexer dielectric filter having an open area free from a conductive layer on the side surface of a dielectric block within a reception area, thus reducing the loading capacitance and increasing the coupling capacitance between neighboring resonators, and thereby producing a desired small-sized duplexer dielectric filter.
2. Description of the Prior Art
As is well known to those skilled in the art and the general public, mobile communication systems using super high frequency band waves have been largely substituted for conventional wire communication systems. Therefore, cellular phones are widely used and are subjected to active research and development to improve their operational performance and achieve the desired compactness, smallness and lightness thereof.
A duplexer filter is designed to commonly transmit and receive signals using one antenna at the same time. Such a duplexer filter comprises a reception filter and a transmission filter. The reception filter passes reception frequency components, but suppresses transmission frequency components. On the other hand, the transmission filter passes transmission frequency components, but suppresses reception frequency components. In order to use such duplexer filters in cellular phones, it is necessary that said duplexer filters be made to occupy a very small space. Such an object may be accomplished by integrated duplexer dielectric filters.
FIG. 1 is a perspective view showing the construction of a conventional integrated duplexer dielectric filter. As shown in the drawing, the conventional integrated duplexer dielectric filter comprises a dielectric block 1 having a generally cubic-shape. This duplexer dielectric filter has two filtering areas: a transmission area 10 and a reception area 20. The dielectric block 1 has an upper surface 3, a lower surface, and a side surface 5. A conductive material is coated on the lower surface and the side surface 5. A series of resonating holes 7 are regularly and parallely formed in the dielectric block 1 in such a way that the holes 7 completely extend from the upper surface 3 to the lower surface and are spaced apart from each other at regular intervals. The resonating holes 7 are entirely coated with a conductive material on their internal surfaces, thereby forming desired resonators.
A conductive pattern 9, having a predetermined size, is formed on the upper surface 3 of the dielectric block 1 at a position around each of the resonating holes 7. Such conductive patterns 9 are connected to the conductive layers on the internal surfaces of the resonating holes 7, thus forming a loading capacitance between the resonating holes 7 and the conductive layer of the side surface 5, and forming a coupling capacitance between neighboring resonators. The resonance frequency of the resonators is determined by both the resonating holes 7 and the loading capacitance, while the coupling capacitance couples the resonators to each other. The transmission area 10 and the reception area 20 of the upper and side surfaces 3 and 5 of the dielectric block 1 are provided with transmission and reception-terminals 12a and 12b for accomplishing the signal transmission and reception operation. An antenna terminal 12c, consisting of a conductive pattern, is formed at a position between the transmission and reception areas 10 and 20. The transmission terminal 12a, the reception terminal 12b and the antenna terminal 12c are insulated from the conductive material disposed on the side surface 5 of the dielectric block by open areas 14a, 14b and 14c, respectively.
FIG. 2 is an equivalent circuit diagram of the duplexer dielectric filter of FIG. 1. In FIG. 2, the reference character xe2x80x9cRxe2x80x9d denotes transmitting lines, each of which is always opened at one end thereof by an associated resonating hole 7 of the dielectric block 1. As described above, he antenna terminal is disposed between the transmission area and the reception area. The elements related to the resonating holes 7 within the transmission area are indicated by the reference labels including the character xe2x80x9ctxe2x80x9d, for example, Cti, Ctij, Rti and Mtij, while the elements related to the resonating holes 7 within the reception area are indicated by the reference labels including the character xe2x80x9crxe2x80x9d, for example, Cri, Crij, Rri and Mrij. The loading capacitance Cti, Cri (i=1, 2, 3), formed between the resonating holes 7 and the conductive layer on the side surface 5 of the dielectric block 1, is connected to the open ends of the signal transmitting lines. A desired resonating circuit is formed by both the signal transmitting lines Rti, RH (i=1, 2, 3) and the loading capacitance.
In a conventional duplexer dielectric filter, it is necessary to accomplish both desired signal transmitting characteristics within a transmission frequency band and desired attenuation characteristics within a low frequency band. The desired transmission characteristics within the transmission frequency band are determined by a coupling of the resonance frequency of the resonators, determined by both the signal transmitting lines Rti, Rri and the loading capacitance Cti, Cri, the coupling capacitance Ctij, Crij (i,j=1, 2, 3), and electromagnetic coupling values Mtij, Mrij (i,j=1, 2, 3). The desired attenuation characteristics within the low frequency band are determined by a coupling. That is, both the attenuation characteristics and the frequency of an attenuation pole are determined by a combination of the coupling capacitance and magnetic coupling values.
In the conventional duplexer dielectric filters, the high frequency band within the transmission area is relatively lower than that of the reception area. Therefore, the electric field effect between the resonating holes is relatively higher within the reception area than that of the transmission area, but the magnetic field effect between the resonating holes is relatively higher in the transmission area than that of the reception area. Therefore, the resonators within the reception area form a capacitance coupling, but the resonators within the transmission area form an inductance coupling.
In such a conventional duplexer dielectric filter of FIG. 1, the determination of the resonance frequency or the coupling between the resonators are changed in accordance with the size of the conductive patterns 9 formed on the upper surface 3 of the dielectric block 1. In other words, the operational characteristics of the duplexer the dielectrc filters are changed in accordance with both the gap between the conductive patterns 9 and the conductive layer of the side surface 5, and the gap between the conductive patterns 9.
As described above, in order to determine the resonance frequency of the duplexer dielectric filter, it is necessary to control the distance between the conductive patterns 9, formed on the upper surface 3 of the dielectric block 1, and the conductive layer formed on the side surface 5 of the dielectric block 1. However, in a conventional duplexer dielectric filter, the resonance frequency within the transmission area is lower than that of the reception area, and so it is necessary to make the loading capacitance within the transmission area higher than that of the reception area. In order to form a high loading capacitance within the transmission area, it is necessary to enlarge the size of the conductive patterns 9 within the transmission area and to complicate the arrangement of those conductive patterns 9.
The length of the signal transmitting lines within the transmission area 10 is equal to that of the reception area 20, and so it is necessary to properly control both the loading capacitance and the coupling capacitance so as to accomplish the desired filtering characteristics of the duplexer dielectric filter. Such a capacitance may be properly controlled by changing the pattern and size of the conductive patterns 9. When the size of the dielectric block 1 is reduced to achieve the desired compactness, smallness and lightness of the duplexer dielectric filter, the following problems may be generated:
When forming the conductive patterns 9 through a screen printing process that is the most typical process, the line width or the line interval is about 150 xcexcm. The maximum loading capacitance is thus undesirably reduced in accordance with a reduction in the printed area for the conductive patterns 9 during such a screen printing process. Therefore, in the case of the transmission terminal, it is necessary to lengthen the signal transmitting line R so as to maintain the loading capacitance at a preset level, although it is desired to reduce the size of the duplexer dielectric filter.
On the other hand, the length of the signal transmitting lines within the reception area 20 is equal to that of the transmission area 10, and so the resonance frequency of the signal transmitting line is reduced in inverse proportion to the length of the signal transmitting line. In such a case, the size of the conductive patterns is reduced. Since the size of the conductive patterns within the reception area is smaller than that of the transmission area due to the coupling capacitance, there is a limit to the possible reduction in the size of the conductive patterns within the reception area. For example, when using a dielectric block that is thinner than a conventional dielectric block, the resonance frequency of the signal transmitting line is reduced, but the gap between the conductive patterns is increased due to the reduction in the size of the conductive patterns. This finally reduces the coupling capacitance. Therefore, it is almost impossible to form a coupling capacitance having a desired value. When the dielectric block is reduced in thickness, the length of the signal transmitting line is increased in accordance with the preset loading capacitance. This finally reduces the market competitiveness of the resulting duplexer dielectric filter, since the production cost of such filters is undesirably increased.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a duplexer dielectric filter, which has an open area free from a conductive layer on the side surface of a dielectric block within a reception area, thus properly controlling both the loading capacitance and the coupling capacitance of a resonator.
Another object of the present invention is to provide a duplexer dielectric filter, which has an open area free from a By conductive layer on the side surface of a dielectric block within a reception area, thus reducing the loading capacitance and increasing the coupling capacitance, and thereby producing a desired small-sized and light duplexer dielectric filter.
In order to accomplish the above object, the present invention provides a duplexer dielectric filter, comprising: a dielectric block having an upper surface, a lower surface, and a side surface, with a conductive material coated on at least a part of the lower and side surfaces; a reception area for filtering a received signal, the reception area comprising at least one resonator including a resonating hole, the resonating hole completely extending from the upper surface to the lower surface of the dielectric block while being at least partially coated with a conductive material on its internal surface; a transmission area for filtering a signal to be transmitted, the transmission area comprising at least one resonator including a resonating hole, the resonating hole completely extending from the upper surface to the lower surface of the dielectric block while being at least partially coated with a conductive material on its internal surface; a transmission terminal for accomplishing a signal transmission operation, the transmission terminal comprising an electrode area formed on the upper and side surfaces of the dielectric block at a position corresponding to the transmission area while being insulated from the conductive material coated on the side surface of the dielectric block; a reception terminal for accomplishing a signal reception operation, the reception terminal comprising an electrode area formed on the upper and side surfaces of the dielectric block at a position corresponding to the reception area while being insulated from the conductive material coated on the side surface of the dielectric block; an antenna terminal arranged between the reception and transmission areas and comprising an electrode area insulated from the conductive material coated on the side surface of the dielectric block; and an open area formed on at least a part of the side surface of the dielectric block at a position corresponding to the reception area while being free from a conductive material, the open area controlling both a coupling capacitance and a loading capacitance of the resonators.
In the above duplexer dielectric filter, the loading capacitance and the coupling capacitance is changed in accordance with the size of the open area defining the gap between the ground electrode and the resonators. The open area may be formed on the side surface of the dielectric block at one position or may be formed at a plurality of positions corresponding to the resonators within the reception area.